Interactions between muscles and nerves are being explored on the basis of changes in the morphological, biochemical, and functional properties of muscle tissue, as induced by certain procedures. Properties and alterations of Z-lines are somewhat emphasized. The procedures include free tenotomy of muscles, tenotomy combined with peripheral denervation, tenotomy combined with denervation and spinal cord section, and tenotomy under circumstances whereby the tenotomized muscle is not permitted to shorten. Other procedures include the repositioning of fast and slow muscles so that fast muscles are required to perform the function of slow muscles, and vice-versa. Minced fast and slow muscle will also be cross transplanted into opposite beds, utilizing the implantation of normal and crossed nerve endings. Through these studies it is hoped that further information concerning the influence of the nervous system on properties of muscle tissue will be gained, as well as information on the possible influence of muscle function on the nervous system.